1. Field of the Invention
This invention relates to an improved static Ward Leonard system in which two sets of thyristor converters are connected in opposition to reversibly energize a D.C. electric motor.
2. Description of the Prior Art
A static Ward Leonard system has heretofore been employed to energize a D.C. electric motor for driving a blooming mill or a thick plate rolling mill which is driven both reversibly and frequently. There are two well known types of static Ward Leonard system, one is directed to a current circulation type and other to a non-current circulation type.
The former current circulation type includes two sets of thyristor converters connected in opposition so that a D.C. electric motor is driven by being energized from the one of the thyristor converters which operates as a rectifier to apply its output D.C. voltage to the motor while the other thyristor converter momentarily operates as an inverter for supplying a regenerative brake to the motor to prepare for reverse rotation of the motor. When a signal for reverse rotation of the motor is applied to a gate control which is provided commonly to both thyristor converters to control gate electrodes thereof, the former converter which has been operating as a rectifier is controlled by the gate control immediately to reduce its D.C. output current and finally to come to a halt. In this event, since the latter converter is retained as an inverter, electric energy from the motor is fed back immediately to an A.C. voltage supply of the latter therethrough to apply the regenerative brake to the motor. The motor is thus controlled to rapidly reduce its speed. After finishing that, the latter converter is transferred from inverter to rectifier, so that the motor is energized therefrom to start reverse rotation.
Although this system was satisfactory in that it has been capable of making the reversible running of the motor smooth, it was unsatisfactory in that one of the two thyristors is retained as an inverter during the time the motor is rotating by being energized from the other thyristor. As a result, there has unsatisfactorily occurred a cross-current flowing from the rectifier to the inverter accompanied by an undesired loss of electric power.
There was, therefore, a need to provide reactors in the anti-parallel connection circuit of converters to prevent or to reduce the cross-current effectively. However, the installation of such reactors has forced the static Ward Leonard system not only to be expensive but also to be large in size.
Because of the above, the aforesaid static Ward Leonard system of noncurrent circulation type has been developed. The system of this type provides two sets of thyristor converters connected in opposition without reactors for the prevention of any cross-current between rectifier and inverter.
In this case, one of the converters is used as a rectifier to apply a D.C. voltage therefrom to the motor to forwardly rotate it while the other converter remains still. As soon as a switchover order signal is applied to a gate control of the system, the gate control forces the electric control angle of lag of the former rectifier to be larger so that the rectifier output current is brought to zero immediately. After that, the other converter starts to operate momentarily as an inverter to absorb electric energy generated from the motor so that the motor speed is effectively reduced under a regenerative effect. As soon as the motor speed has reached zero, the other converter is transferred from inverter to rectifier to energize the motor therefrom to rotate it in reverse.
As described above, in accordance with this system, it is readily understood that the system can be manufactured not only to be inexpensive, but also to be smaller in size because of the lack of reactors for reducing cross-current.
However, in this system, it is very difficult to overcome the rushing current which runs from the motor into the converter that has just become an inverter. This results from the fact that the amplitude of inverter voltage does not coincide with that of the motor armature voltage so that the latter is considerably higher than the former when the switchover has been made between converters.
In accordance with the invention, the rushing current has been reduced by having the amplitude of the inverter voltage controlled by the gate control so as to be as equal as possible to the amplitude of the rectifier voltage at the instant of switchover between converters.
This has been achieved by a gate control which includes a memory device therein to memorize a motor speed controlling signal just before beginning switchover between two converters so that the gate control angle of lead for the converter which has just become the inverter is determined by the memorized controlling signal.
Although this has been somewhat satisfactory, the amplitude of rushing current flowing from motor to inverter has remained undesireably high in spite of the use of a gate control.
This was caused by a specific gate control angle of lead of the inverter just after the switchover. Although this will be described in detail hereinafter, an outline of this will be described here. If the gate control angle of lead of the inverter has been selected so as to be small enough to cause discontinuance of the inverter current when the switch over has just been carried out between converters, the inverter voltage will become insufficient in its amplitude as compared with that of the rectifier voltage of the motor armature before switchover. The rushing current will thus flow from the motor into the converter operating as an inverter which may destroy the thyristor elements therein.